Ships and boats have developed alongside humanity. In armed conflict and in daily life they have become an integral part of modern commercial and military systems. Fishing boats are used by millions of fishermen throughout the world. Military forces operate vessels for naval warfare and to transport and support forces ashore. Commercial vessels, nearly 35,000 in number, carried 7.4 billion tons of cargo in 2007.[1] As of 2011, there are about 104,304 ships with IMO numbers in the world.[2]

Ships were always a key in history's great explorations and scientific and technological development. Navigators such as Zheng He spread such inventions as the compass and gunpowder. Ships have been used for such purposes as colonization and the slave trade, and have served scientific, cultural, and humanitarian needs. After the 16th century, new crops that had come from and to the Americas via the European seafarers significantly contributed to the world population growth.[3]Ship transport has shaped the world's economy into today's energy-intensive pattern.

Ships can usually be distinguished from boats based on size and the ship's ability to operate independently for extended periods.[4] A commonly used rule of thumb is that if one vessel can carry another, the larger of the two is a ship.[5]Dinghies are carried on sailing yachts as small as 35 feet (10.67 m), clearly not ships; this rule of thumb is not foolproof.

A number of large vessels are usually referred to as boats. Submarines are a prime example.[6] Other types of large vessel which are traditionally called boats are the Great Lakes freighter, the riverboat, and the ferryboat.[citation needed] Though large enough to carry their own boats and heavy cargoes, these vessels are designed for operation on inland or protected coastal waters.

In most maritime traditions ships have individual names, and modern ships may belong to a ship class often named after its first ship. In English, a ship is traditionally referred to as "she", even if named after a man, but this is not universal usage; some journalistic style guides advise using "it" as referring to ships with female pronouns can be seen as offensive and out dated.[7][8]

The first known vessels date back about 10,000 years ago, but could not be described as ships. The first navigators began to use animal skins or woven fabrics as sails. Affixed to the top of a pole set upright in a boat, these sails gave early ships range. This allowed men to explore widely, allowing for the settlement of Oceania for example (about 3,000 years ago).

It is known that ancient Nubia/Axum traded with India, and there is evidence that ships from Northeast Africa may have sailed back and forth between India/Sri Lanka and Nubia trading goods and even to Persia, Himyar and Rome.[13]Aksum was known by the Greeks for having seaports for ships from Greece and Yemen.[14]

A panel found at Mohenjodaro depicted a sailing craft. Vessels were of many types; their construction is vividly described in the Yukti Kalpa Taru, an ancient Indian text on shipbuilding. This treatise gives a technical exposition on the techniques of shipbuilding. It sets forth minute details about the various types of ships, their sizes, and the materials from which they were built. The Yukti Kalpa Taru sums up in a condensed form all the available information. The Yukti Kalpa Taru gives sufficient information and dates to prove that, in ancient times, Indian shipbuilders had a good knowledge of the materials which were used in building ships. In addition to describing the qualities of the different types of wood and their suitability for shipbuilding, the Yukti Kalpa Taru gives an elaborate classification of ships based on their size.

The oldest discovered sea faring hulled boat is the Egyptian Uluburun shipwreck off the coast of Turkey, dating back to 1300 BC.[16]

In China, by the time of the Zhou Dynasty ship technologies such as stern mounted rudders were developed, and by the Han Dynasty, a well kept naval fleet was an integral part of the military. Ship technology advanced to the point where by the medieval period, water tight compartments were developed.

The Swahili people had various extensive trading ports dotting the coast of medieval East Africa and Great Zimbabwe had extensive trading contacts with Central Africa, and likely also imported goods brought to Africa through the Southeast African shore trade of Kilwa in modern-day Tanzania.[19]

It is known by historians that at its height the Mali Empire built a large naval fleet under Emperor Mansa Musa in the late 13th and early 14th century.[20] Arabic sources describe what some consider to be visits to the New World by a Mali fleet in 1311.[21]

Before the introduction of the compass, celestial navigation was the main method for navigation at sea. In China, early versions of the magnetic compass were being developed and used in navigation between 1040 and 1117.[22] The true mariner's compass, using a pivoting needle in a dry box, was developed in Europe no later than 1300.[23][24]

A 3D model of the basic hull structure of a Venetian "galley of Flanders", a large Mediterranean trading vessel of the 15th century. The reconstruction by archaeologist Courtney Higgins is based on measurements given in contemporary ship treatises.[25]

Towards the end of the 14th century, ships like the carrack began to develop towers on the bow and stern. These towers decreased the vessel's stability, and in the 15th century, the caravel, designed by the Portuguese, based on the Arabic qarib which could sail closer to the wind, became more widely used. The towers were gradually replaced by the forecastle and sterncastle, as in the carrack Santa María of Christopher Columbus. This increased freeboard allowed another innovation: the freeing port, and the artillery associated with it.

In the 16th century, the use of freeboard and freeing ports became widespread on galleons. The English modified their vessels to maximize their firepower and demonstrated the effectiveness of their doctrine, in 1588, by defeating the Spanish Armada.

At this time, ships were developing in Asia in much the same way as Europe. Japan used defensive naval techniques in the Mongol invasions of Japan in 1281. It is likely that the Mongols of the time took advantage of both European and Asian shipbuilding techniques. During the 15th century, China's Ming Dynasty assembled one of the largest and most powerful naval fleet in the world for the diplomatic and power projection voyages of Zheng He. Elsewhere in Japan in the 15th century, one of the world's first iron-clads, "Tekkōsen" (鉄甲船), literally meaning "iron ships",[26] was also developed. In Japan, during the Sengoku era from the fifteenth to 17th century, the great struggle for feudal supremacy was fought, in part, by coastal fleets of several hundred boats, including the atakebune.

Fifty years before Christopher Columbus, Chinese navigator Zheng He traveled the world at the head of what was for the time a huge armada. The largest of his ships had nine masts, were 130 metres (430 ft) long and had a beam of 55 metres (180 ft). His fleet carried 30,000 men aboard 70 vessels, with the goal of bringing glory to the Chinese emperor.

Parallel to the development of warships, ships in service of marine fishery and trade also developed in the period between antiquity and the Renaissance. Still primarily a coastal endeavor, fishing is largely practiced by individuals with little other money using small boats.

Maritime trade was driven by the development of shipping companies with significant financial resources. Canal barges, towed by draft animals on an adjacent towpath, contended with the railway up to and past the early days of the industrial revolution. Flat-bottomed and flexible scow boats also became widely used for transporting small cargoes. Mercantile trade went hand-in-hand with exploration, self-financed by the commercial benefits of exploration.

During the first half of the 18th century, the French Navy began to develop a new type of vessel known as a ship of the line, featuring seventy-four guns. This type of ship became the backbone of all European fighting fleets. These ships were 56 metres (184 ft) long and their construction required 2,800 oak trees and 40 kilometres (25 mi) of rope; they carried a crew of about 800 sailors and soldiers.

Ship designs stayed fairly unchanged until the late 19th century. The industrial revolution, new mechanical methods of propulsion, and the ability to construct ships from metal triggered an explosion in ship design. Factors including the quest for more efficient ships, the end of long running and wasteful maritime conflicts, and the increased financial capacity of industrial powers created an avalanche of more specialized boats and ships. Ships built for entirely new functions, such as firefighting, rescue, and research, also began to appear.

In light of this, classification of vessels by type or function can be difficult. Even using very broad functional classifications such as fishery, trade, military, and exploration fails to classify most of the old ships. This difficulty is increased by the fact that the terms such as sloop and frigate are used by old and new ships alike, and often the modern vessels sometimes have little in common with their predecessors.

The size of the world's fishing fleet is more difficult to estimate. The largest of these are counted as commercial vessels, but the smallest are legion. Fishing vessels can be found in most seaside villages in the world. As of 2004, the United Nations Food and Agriculture Organization estimated 4 million fishing vessels were operating worldwide.[41] The same study estimated that the world's 29 million fishermen[42] caught 85,800,000 tonnes (84,400,000 long tons; 94,600,000 short tons) of fish and shellfish that year.[43]

Because ships are constructed using the principles of naval architecture that require same structural components, their classification is based on their function such as suggested by Paulet and Presles.,[44] which requires modification of the components. The categories accepted in general by naval architects are:

Many types of boats are designed for inland and coastal waterways. These are the vessels that trade upon the lakes, rivers and canals.

Barges are a prime example of inland vessels. Flat-bottomed boats built to transport heavy goods, most barges are not self-propelled and need to be moved by tugboats towing or towboats pushing them. Barges towed along canals by draft animals on an adjacent towpath contended with the railway in the early industrial revolution but were out competed in the carriage of high value items because of the higher speed, falling costs, and route flexibility of rail transport.

Lake freighters, also called lakers, are cargo vessels that ply the Great Lakes. The most well-known is the SS Edmund Fitzgerald, the latest major vessel to be wrecked on the Lakes. These vessels are traditionally called boats, not ships. Visiting ocean-going vessels are called "salties." Because of their additional beam, very large salties are never seen inland of the Saint Lawrence Seaway. Because the smallest of the Soo Locks is larger than any Seaway lock, salties that can pass through the Seaway may travel anywhere in the Great Lakes. Because of their deeper draft, salties may accept partial loads on the Great Lakes, "topping off" when they have exited the Seaway. Similarly, the largest lakers are confined to the Upper Lakes (Superior, Michigan, Huron, Erie) because they are too large to use the Seaway locks, beginning at the Welland Canal that bypasses the Niagara River.

Since the freshwater lakes are less corrosive to ships than the salt water of the oceans, lakers tend to last much longer than ocean freighters. Lakers older than 50 years are not unusual, and as of 2005, all were over 20 years of age.[45]

The SS St. Marys Challenger, built in 1906 as the William P Snyder, was the oldest laker still working on the Lakes until its conversion into a barge starting in 2013. Similarly, the E.M. Ford, built in 1898 as the Presque Isle, was sailing the lakes 98 years later in 1996. As of 2007 the Ford was still afloat as a stationary transfer vessel at a riverside cement silo in Saginaw, Michigan.

Passenger ships range in size from small river ferries to very large cruise ships. This type of vessel includes ferries, which move passengers and vehicles on short trips; ocean liners, which carry passengers from one place to another; and cruise ships, which carry passengers on voyages undertaken for pleasure, visiting several places and with leisure activities on board, often returning them to the port of embarkation. Riverboats and inland ferries are specially designed to carry passengers, cargo, or both in the challenging river environment. Rivers present special hazards to vessels. They usually have varying water flows that alternately lead to high speed water flows or protruding rock hazards. Changing siltation patterns may cause the sudden appearance of shoal waters, and often floating or sunken logs and trees (called snags) can endanger the hulls and propulsion of riverboats. Riverboats are generally of shallow draft, being broad of beam and rather square in plan, with a low freeboard and high topsides. Riverboats can survive with this type of configuration as they do not have to withstand the high winds or large waves that are seen on large lakes, seas, or oceans.

Fishing vessels are a subset of commercial vessels, but generally small in size and often subject to different regulations and classification. They can be categorized by several criteria: architecture, the type of fish they catch, the fishing method used, geographical origin, and technical features such as rigging. As of 2004, the world's fishing fleet consisted of some 4 million vessels.[41] Of these, 1.3 million were decked vessels with enclosed areas and the rest were open vessels.[41] Most decked vessels were mechanized, but two-thirds of the open vessels were traditional craft propelled by sails and oars.[41] More than 60% of all existing large fishing vessels[48] were built in Japan, Peru, the Russian Federation, Spain or the United States of America.[49]

A weather ship was a ship stationed in the ocean as a platform for surface and upper air meteorological observations for use in marine weather forecasting. Surface weather observations were taken hourly, and four radiosonde releases occurred daily.[50] It was also meant to aid in search and rescue operations and to support transatlantic flights.[50][51] Proposed as early as 1927 by the aviation community,[52] the establishment of weather ships proved to be so useful during World War II that the International Civil Aviation Organization (ICAO) established a global network of weather ships in 1948, with 13 to be supplied by the United States.[51] This number was eventually negotiated down to nine.[53]

The weather ship crews were normally at sea for three weeks at a time, returning to port for 10 day stretches.[50] Weather ship observations proved to be helpful in wind and wave studies, as they did not avoid weather systems like other ships tended to for safety reasons.[54] They were also helpful in monitoring storms at sea, such as tropical cyclones.[55] The removal of a weather ship became a negative factor in forecasts leading up to the Great Storm of 1987.[56] Beginning in the 1970s, their role became largely superseded by weather buoys due to the ships' significant cost.[57] The agreement of the use of weather ships by the international community ended in 1990. The last weather ship was Polarfront, known as weather station M ("Mike"), which was put out of operation on 1 January 2010. Weather observations from ships continue from a fleet of voluntary merchant vessels in routine commercial operation.

Fast combat vessels such as cruisers and destroyers usually have fine hulls to maximize speed and maneuverability.[60] They also usually have advanced marine electronics and communication systems, as well as weapons.

Some components exist in vessels of any size and purpose. Every vessel has a hull of sorts. Every vessel has some sort of propulsion, whether it's a pole, an ox, or a nuclear reactor. Most vessels have some sort of steering system. Other characteristics are common, but not as universal, such as compartments, holds, a superstructure, and equipment such as anchors and winches.

A ship's hull endures harsh conditions at sea, as illustrated by this reefer ship in bad weather.

For a ship to float, its weight must be less than that of the water displaced by the ship's hull.[61] There are many types of hulls, from logs lashed together to form a raft to the advanced hulls of America's Cup sailboats. A vessel may have a single hull (called a monohull design), two in the case of catamarans, or three in the case of trimarans. Vessels with more than three hulls are rare, but some experiments have been conducted with designs such as pentamarans. Multiple hulls are generally parallel to each other and connected by rigid arms.

Hulls have several elements. The bow is the foremost part of the hull. Many ships feature a bulbous bow. The keel is at the very bottom of the hull, extending the entire length of the ship. The rear part of the hull is known as the stern, and many hulls have a flat back known as a transom. Common hull appendages include propellers for propulsion, rudders for steering, and stabilizers to quell a ship's rolling motion. Other hull features can be related to the vessel's work, such as fishing gear and sonar domes.

Hulls are subject to various hydrostatic and hydrodynamic constraints. The key hydrostatic constraint is that it must be able to support the entire weight of the boat, and maintain stability even with often unevenly distributed weight. Hydrodynamic constraints include the ability to withstand shock waves, weather collisions and groundings.

Older ships and pleasure craft often have or had wooden hulls. Steel is used for most commercial vessels. Aluminium is frequently used for fast vessels, and composite materials are often found in sailboats and pleasure craft. Some ships have been made with concrete hulls.

Propulsion systems for ships fall into three categories: human propulsion, sailing, and mechanical propulsion. Human propulsion includes rowing, which was used even on large galleys. Propulsion by sail generally consists of a sail hoisted on an erect mast, supported by stays and spars and controlled by ropes. Sail systems were the dominant form of propulsion until the 19th century. They are now generally used for recreation and competition, although experimental sail systems, such as the turbosails, rotorsails, and wingsails have been used on larger modern vessels for fuel savings.

In addition to traditional fixed and controllable pitch propellers there are many specialized variations, such as contra-rotating and nozzle-style propellers. Most vessels have a single propeller, but some large vessels may have up to four propellers supplemented with transverse thrusters for maneuvring at ports. The propeller is connected to the main engine via a propeller shaft and, in case of medium- and high-speed engines, a reduction gearbox. Some modern vessels have a diesel-electric powertrain in which the propeller is turned by an electric motor powered by the ship's generators.

For ships with independent propulsion systems for each side, such as manual oars or some paddles,[62] steering systems may not be necessary. In most designs, such as boats propelled by engines or sails, a steering system becomes necessary. The most common is a rudder, a submerged plane located at the rear of the hull. Rudders are rotated to generate a lateral force which turns the boat. Rudders can be rotated by a tiller, manual wheels, or electro-hydraulic systems. Autopilot systems combine mechanical rudders with navigation systems. Ducted propellers are sometimes used for steering.

Larger boats and ships generally have multiple decks and compartments. Separate berthings and heads are found on sailboats over about 25 feet (7.6 m). Fishing boats and cargo ships typically have one or more cargo holds. Most larger vessels have an engine room, a galley, and various compartments for work. Tanks are used to store fuel, engine oil, and fresh water. Ballast tanks are equipped to change a ship's trim and modify its stability.

Superstructures are found above the main deck. On sailboats, these are usually very low. On modern cargo ships, they are almost always located near the ship's stern. On passenger ships and warships, the superstructure generally extends far forward.

Shipboard equipment varies from ship to ship depending on such factors as the ship's era, design, area of operation, and purpose. Some types of equipment that are widely found include:

Masts can be the home of antennas, navigation lights, radar transponders, fog signals, and similar devices often required by law.

Ground tackle includes equipment such as mooring winches, windlasses, and anchors. Anchors are used to moor ships in shallow water. They are connected to the ship by a rope or chain. On larger vessels, the chain runs through a hawsepipe.

Cargo equipment such as cranes and cargo booms are used to load and unload cargo and ship's stores.

Boats and ships are kept on (or slightly above) the water in three ways:

For most vessels, known as displacement vessels, the vessel's weight is offset by that of the water displaced by the hull.

For planing ships and boats, such as the hydrofoil, the lift developed by the movement of the foil through the water increases with the vessel's speed, until the vessel is foilborne.

For non-displacement craft such as hovercraft and air-cushion vehicles, the vessel is suspended over the water by a cushion of high-pressure air it projects downwards against the surface of the water.

A vessel is in equilibrium when the upwards and downwards forces are of equal magnitude. As a vessel is lowered into the water its weight remains constant but the corresponding weight of water displaced by its hull increases. When the two forces are equal, the boat floats. If weight is evenly distributed throughout the vessel, it floats without trim or heel.

A vessel's stability is considered in both this hydrostatic sense as well as a hydrodynamic sense, when subjected to movement, rolling and pitching, and the action of waves and wind. Stability problems can lead to excessive pitching and rolling, and eventually capsizing and sinking.

The advance of a vessel through water is resisted by the water. This resistance can be broken down into several components, the main ones being the friction of the water on the hull and wave making resistance. To reduce resistance and therefore increase the speed for a given power, it is necessary to reduce the wetted surface and use submerged hull shapes that produce low amplitude waves. To do so, high-speed vessels are often more slender, with fewer or smaller appendages. The friction of the water is also reduced by regular maintenance of the hull to remove the sea creatures and algae that accumulate there. Antifouling paint is commonly used to assist in this. Advanced designs such as the bulbous bow assist in decreasing wave resistance.

A simple way of considering wave-making resistance is to look at the hull in relation to its wake. At speeds lower than the wave propagation speed, the wave rapidly dissipates to the sides. As the hull approaches the wave propagation speed, however, the wake at the bow begins to build up faster than it can dissipate, and so it grows in amplitude. Since the water is not able to "get out of the way of the hull fast enough", the hull, in essence, has to climb over or push through the bow wave. This results in an exponential increase in resistance with increasing speed.

When the vessel exceeds a speed/length ratio of 0.94, it starts to outrun most of its bow wave, and the hull actually settles slightly in the water as it is now only supported by two wave peaks. As the vessel exceeds a speed/length ratio of 1.34, the hull speed, the wavelength is now longer than the hull, and the stern is no longer supported by the wake, causing the stern to squat, and the bow rise. The hull is now starting to climb its own bow wave, and resistance begins to increase at a very high rate. While it is possible to drive a displacement hull faster than a speed/length ratio of 1.34, it is prohibitively expensive to do so. Most large vessels operate at speed/length ratios well below that level, at speed/length ratios of under 1.0.

For large projects with adequate funding, hydrodynamic resistance can be tested experimentally in a hull testing pool or using tools of computational fluid dynamics.

Vessels are also subject to ocean surface waves and sea swell as well as effects of wind and weather. These movements can be stressful for passengers and equipment, and must be controlled if possible. The rolling movement can be controlled, to an extent, by ballasting or by devices such as fin stabilizers. Pitching movement is more difficult to limit and can be dangerous if the bow submerges in the waves, a phenomenon called pounding. Sometimes, ships must change course or speed to stop violent rolling or pitching.

How it has been convincingly shown in scientific studies of the 21st century,[63][64] controllability of some vessels decreases dramatically in some cases that are conditioned by effects of the bifurcation memory. This class of vessels includes ships with high manoeuvring capabilities, aircraft and controlled underwater vehicles designed to be unstable in steady-state motion that are interesting in terms of applications. These features must be considered in designing ships and in their control in critical situations.

A ship will pass through several stages during its career. The first is usually an initial contract to build the ship, the details of which can vary widely based on relationships between the shipowners, operators, designers and the shipyard. Then, the design phase carried out by a naval architect. Then the ship is constructed in a shipyard. After construction, the vessel is launched and goes into service. Ships end their careers in a number of ways, ranging from shipwrecks to service as a museum ship to the scrapyard.

A vessel's design starts with a specification, which a naval architect uses to create a project outline, assess required dimensions, and create a basic layout of spaces and a rough displacement. After this initial rough draft, the architect can create an initial hull design, a general profile and an initial overview of the ship's propulsion. At this stage, the designer can iterate on the ship's design, adding detail and refining the design at each stage.

The designer will typically produce an overall plan, a general specification describing the peculiarities of the vessel, and construction blueprints to be used at the building site. Designs for larger or more complex vessels may also include sail plans, electrical schematics, and plumbing and ventilation plans.

As environmental laws are becoming more strict, ship designers need to create their design in such a way that the ship, when it nears its end-of-term, can be disassmbled or disposed easily and that waste is reduced to a minimum.

Ship construction takes place in a shipyard, and can last from a few months for a unit produced in series, to several years to reconstruct a wooden boat like the frigate Hermione, to more than 10 years for an aircraft carrier. During World War II, the need for cargo ships was so urgent that construction time for Liberty Ships went from initially eight months or longer, down to weeks or even days. Builders employed production line and prefabrication techniques such as those used in shipyards today.[65][66][67]

Hull materials and vessel size play a large part in determining the method of construction. The hull of a mass-produced fiberglass sailboat is constructed from a mold, while the steel hull of a cargo ship is made from large sections welded together as they are built.

Generally, construction starts with the hull, and on vessels over about 30 meters (98 ft), by the laying of the keel. This is done in a drydock or on land. Once the hull is assembled and painted, it is launched. The last stages, such as raising the superstructure and adding equipment and accommodation, can be done after the vessel is afloat.

Once completed, the vessel is delivered to the customer. Ship launching is often a ceremony of some significance, and is usually when the vessel is formally named. A typical small rowboat can cost under US$100, $1,000 for a small speedboat, tens of thousands of dollars for a cruising sailboat, and about $2,000,000 for a Vendée Globe class sailboat. A 25 meters (82 ft) trawler may cost $2.5 million, and a 1,000-person-capacity high-speed passenger ferry can cost in the neighborhood of $50 million. A ship's cost partly depends on its complexity: a small, general cargo ship will cost $20 million, a Panamax-sized bulk carrier around $35 million, a supertanker around $105 million and a large LNG carrier nearly $200 million. The most expensive ships generally are so because of the cost of embedded electronics: a Seawolf-classsubmarine costs around $2 billion, and an aircraft carrier goes for about $3.5 billion.

Ships undergo nearly constant maintenance during their career, whether they be underway, pierside, or in some cases, in periods of reduced operating status between charters or shipping seasons.

Most ships, however, require trips to special facilities such as a drydock at regular intervals. Tasks often done at drydock include removing biological growths on the hull, sandblasting and repainting the hull, and replacing sacrificial anodes used to protect submerged equipment from corrosion. Major repairs to the propulsion and steering systems as well as major electrical systems are also often performed at dry dock.

Most ocean-going cargo ships have a life expectancy of between 20 and 30 years. A sailboat made of plywood or fiberglass can last between 30 and 40 years. Solid wooden ships can last much longer but require regular maintenance. Carefully maintained steel-hulled yachts can have a lifespan of over 100 years.

One can measure ships in terms of overall length, length of the ship at the waterline, beam (breadth), depth (distance between the crown of the weather deck and the top of the keelson), draft (distance between the highest waterline and the bottom of the ship) and tonnage. A number of different tonnage definitions exist and are used when describing merchant ships for the purpose of tolls, taxation, etc.

In Britain until Samuel Plimsoll's Merchant Shipping Act of 1876, ship-owners could load their vessels until their decks were almost awash, resulting in a dangerously unstable condition. Anyone who signed on to such a ship for a voyage and, upon realizing the danger, chose to leave the ship, could end up in jail. Plimsoll, a Member of Parliament, realised the problem and engaged some engineers to derive a fairly simple formula to determine the position of a line on the side of any specific ship's hull which, when it reached the surface of the water during loading of cargo, meant the ship had reached its maximum safe loading level. To this day, that mark, called the "Plimsoll Line", exists on ships' sides, and consists of a circle with a horizontal line through the centre. On the Great Lakes of North America the circle is replaced with a diamond. Because different types of water (summer, fresh, tropical fresh, winter north Atlantic) have different densities, subsequent regulations required painting a group of lines forward of the Plimsoll mark to indicate the safe depth (or freeboard above the surface) to which a specific ship could load in water of various densities. Hence the "ladder" of lines seen forward of the Plimsoll mark to this day. This is called the "freeboard mark" or "load line mark" in the marine industry.

Ship pollution is the pollution of air and water by shipping. It is a problem that has been accelerating as trade has become increasingly globalized, posing an increasing threat to the world’s oceans and waterways as globalization continues. It is expected that, “...shipping traffic to and from the USA is projected to double by 2020."[69] Because of increased traffic in ocean ports, pollution from ships also directly affects coastal areas. The pollution produced affects biodiversity, climate, food, and human health. However, the degree to which humans are polluting and how it affects the world is highly debated and has been a hot international topic for the past 30 years.

Oil spills have devastating effects on the environment. Crude oil contains polycyclic aromatic hydrocarbons (PAHs) which are very difficult to clean up, and last for years in the sediment and marine environment.[71] Marine species constantly exposed to PAHs can exhibit developmental problems, susceptibility to disease, and abnormal reproductive cycles.

By the sheer amount of oil carried, modern oil tankers must be considered something of a threat to the environment. An oil tanker can carry 2 million barrels (318,000 m3) of crude oil, or 84,000,000 US gallons (69,940,000 imp gal; 318,000,000 L). This is more than six times the amount spilled in the widely known Exxon Valdez incident. In this spill, the ship ran aground and dumped 10,800,000 US gallons (8,993,000 imp gal; 40,880,000 L) of oil into the ocean in March 1989. Despite efforts of scientists, managers, and volunteers, over 400,000 seabirds, about 1,000 sea otters, and immense numbers of fish were killed.[71]

The International Tanker Owners Pollution Federation has researched 9,351 accidental spills since 1974.[72] According to this study, most spills result from routine operations such as loading cargo, discharging cargo, and taking on fuel oil.[72] 91% of the operational oil spills were small, resulting in less than 7 tons per spill.[72] Spills resulting from accidents like collisions, groundings, hull failures, and explosions are much larger, with 84% of these involving losses of over 700 tons.[72]

Following the Exxon Valdez spill, the United States passed the Oil Pollution Act of 1990 (OPA-90), which included a stipulation that all tankers entering its waters be double-hulled by 2015. Following the sinkings of the Erika (1999) and Prestige (2002), the European Union passed its own stringent anti-pollution packages (known as Erika I, II, and III), which require all tankers entering its waters to be double-hulled by 2010. The Erika packages are controversial because they introduced the new legal concept of "serious negligence".[73]

When a large vessel such as a container ship or an oil tanker unloads cargo, seawater is pumped into other compartments in the hull to help stabilize and balance the ship. During loading, this ballast water is pumped out from these compartments.[74]

One of the problems with ballast water transfer is the transport of harmful organisms. Meinesz[75] believes that one of the worst cases of a single invasive species causing harm to an ecosystem can be attributed to a seemingly harmless jellyfish. Mnemiopsis leidyi, a species of comb jellyfish that inhabits estuaries from the United States to the Valdés peninsula in Argentina along the Atlantic coast, has caused notable damage in the Black Sea. It was first introduced in 1982, and thought to have been transported to the Black Sea in a ship’s ballast water. The population of the jellyfish shot up exponentially and, by 1988, it was wreaking havoc upon the local fishing industry. "The anchovy catch fell from 204,000 tonnes (225,000 short tons; 201,000 long tons) in 1984 to 200 tonnes (220 short tons; 197 long tons) in 1993; sprat from 24,600 tonnes (27,100 short tons; 24,200 long tons) in 1984 to 12,000 tonnes (13,200 short tons; 11,800 long tons) in 1993; horse mackerel from 4,000 tonnes (4,410 short tons; 3,940 long tons) in 1984 to zero in 1993."[75] Now that the jellyfish have exhausted the zooplankton, including fish larvae, their numbers have fallen dramatically, yet they continue to maintain a stranglehold on the ecosystem. Recently the jellyfish have been discovered in the Caspian Sea. Invasive species can take over once occupied areas, facilitate the spread of new diseases, introduce new genetic material, alter landscapes and jeopardize the ability of native species to obtain food. "On land and in the sea, invasive species are responsible for about 137 billion dollars in lost revenue and management costs in the U.S. each year."[71]

Ballast and bilge discharge from ships can also spread human pathogens and other harmful diseases and toxins potentially causing health issues for humans and marine life alike.[76] Discharges into coastal waters, along with other sources of marine pollution, have the potential to be toxic to marine plants, animals, and microorganisms, causing alterations such as changes in growth, disruption of hormone cycles, birth defects, suppression of the immune system, and disorders resulting in cancer, tumors, and genetic abnormalities or even death.[71]

Exhaust emissions from ships are considered to be a significant source of air pollution. “Seagoing vessels are responsible for an estimated 14 percent of emissions of nitrogen from fossil fuels and 16 percent of the emissions of sulfur from petroleum uses into the atmosphere.”[71] In Europe ships make up a large percentage of the sulfur introduced to the air, “...as much sulfur as all the cars, lorries and factories in Europe put together.”[77] “By 2010, up to 40% of air pollution over land could come from ships.”[77] Sulfur in the air creates acid rain which damages crops and buildings. When inhaled sulfur is known to cause respiratory problems and increase the risk of a heart attack.[77]

Ship breaking or ship demolition is a type of ship disposal involving the breaking up of ships for scraprecycling, with the hulls being discarded in ship graveyards. Most ships have a lifespan of a few decades before there is so much wear that refitting and repair becomes uneconomical. Ship breaking allows materials from the ship, especially steel, to be reused.

In addition to steel and other useful materials, however, ships (particularly older vessels) can contain many substances that are banned or considered dangerous in developed countries. Asbestos and polychlorinated biphenyls (PCBs) are typical examples. Asbestos was used heavily in ship construction until it was finally banned in most of the developed world in the mid 1980s. Currently, the costs associated with removing asbestos, along with the potentially expensive insurance and health risks, have meant that ship-breaking in most developed countries is no longer economically viable. Removing the metal for scrap can potentially cost more than the scrap value of the metal itself. In most of the developing world, however, shipyards can operate without the risk of personal injury lawsuits or workers' health claims, meaning many of these shipyards may operate with high health risks. Furthermore, workers are paid very low rates with no overtime or other allowances. Protective equipment is sometimes absent or inadequate. Dangerous vapors and fumes from burning materials can be inhaled, and dusty asbestos-laden areas around such breakdown locations are commonplace.

Aside from the health of the yard workers, in recent years, ship breaking has also become an issue of major environmental concern. Many developing nations, in which ship breaking yards are located, have lax or no environmental law, enabling large quantities of highly toxic materials to escape into the environment and causing serious health problems among ship breakers, the local population and wildlife. Environmental campaign groups such as Greenpeace have made the issue a high priority for their campaigns.[78]

A floating boat displaces its weight in water. The material of the boat hull may be denser than water, but if this is the case then it forms only the outer layer. If the boat floats, the mass of the boat (plus contents) as a whole divided by the volume below the waterline is equal to the density of water (1 kg/l). If weight is added to the boat, the volume below the waterline will increase to keep the weight balance equal, and so the boat sinks a little to compensate.

^The earliest known Egyptian boats date to 3000 BC and were found in Abydos in 1991. They consisted of planks joined by ropes passing through mortises. Similar boats dating to 2600 BC were found in 1954 and 1987 in pits at the Great Pyramid of Khufu in Giza. In 1894, Egyptian boats composed of planks joined by mortises and tenons were found in Dashur. See: ABC.se

^Anzovin, item # 5393, page 385 Reference to a ship with a name appears in an inscription of 2613 BCE that recounts the shipbuilding achievements of the fourth-dynasty Egyptian pharaoh Sneferu. He was recorded as the builder of a cedarwood vessel called "Praise of the Two Lands."

^Almost all paddle steamers had a single engine with their paddles permanently coupled, without any clutches, and so could not be used for steering. Only a few examples with separate engines were steerable. The Royal Navy however operated diesel-electric harbour tugs with paddles into the 1970s, for their superior maneuverability.

^Sawyer, L.A. and Mitchell, W. H. The Liberty Ships: The History of the "Emergency" Type Cargo Ships Constructed in the United States During the Second World War, pp. 7-10, 2nd Edition, Lloyd's of London Press Ltd., London, England, 1985. ISBN 1-85044-049-2.

^National Research Council, Committee on the Ocean's Role in Human Health, Ocean Studies Board, Commission on Geosciences, Environment, and Resources. (1999). From monsoons to microbes: understanding the ocean's role in human health. Washington, D.C.: National Academy Press